The invention is directed to fiber optic sensors, and more particularly to controlling the scale factor of fiber optic sensors, such as gyroscopes and current sensors.
Interferometric fiber optic sensors have found extensive use in the art and may include Fiber Optic Gyroscopes (FOG)s and Fiber Optic Current sensors (FOC)s. In general, light emitted from a suitable light source may pass through a polarizer and is split by a coupler into two approximately equal intensity, counter-propagating beams which traverse the sensor coil. The two light beams exiting the coil may then recombine at the coupler, where they may interfere as a result of a phase shift between the counter-propagating beams. In FOGs, the phase shift results from rotation of the sensor coil. In FOCs, a xcex/4 waveplate may be placed near each end of sensor coil, and the phase shift results from the magnetic field of a current carrying conductor passing through the sensor coil. The recombined light beam may then pass through the polarizer a second time in the opposite direction, and half of the light may be directed to a photodetector by a second coupler. The phase shift detected is related to the rotation rate in the case of a FOG, or to the electric current within the conductor in the case of a FOC, by a scale factor.
FOGs may be operated as a xe2x80x9cclosed loopxe2x80x9d or xe2x80x9copen loopxe2x80x9d system. An open loop fiber optic sensor, such as an open loop FOG, has the advantage of simplicity and low cost compared to a closed loop configuration. On the other hand, closed loop FOGs have the advantage of excellent bias stability, linearity and scale factor stability, although some of these characteristics require thermal modeling on an individual unit basis. However, closed loop FOGs require a wide bandwidth modulator device, while the open loop FOG requires only a single frequency modulator. Conversely, the open loop gyroscope has a sinusoidal response to rotation, and the scale factor is dependent on both the optical intensity at the detector and the modulation depth. These considerations have limited the performance of most open loop gyroscopes.
The theory of open loop gyroscopes is known in the art. The direction of rotation can be determined and the sensitivity optimized by applying a phase modulation at a frequency fm to the light propagating in the fiber optic sensing coil by means of a piezoelectric transducer. Alternatively, other methods of modulating the phase difference, for example, using an electro-optic material such as lithium niobate and/or non-linear polymers, may be employed. The Sagnac interferometer converts this modulation into a detected output signal represented by a series of Bessel functions corresponding to harmonics of the phase modulation frequency fm. The odd harmonics are proportional to the sine of the rotation rate, while the even harmonics are proportional to the cosine of the rotation rate. All of the information required to determine the rotation rate and to linearize and stabilize the scale factor can be extracted from the signal at the fundamental phase modulation frequency and the signals at a limited number of the harmonic frequencies.
Conventional FOG systems process the signal in analog form, which represents an approximation of an analytic approach. A FOG can be operated with a low Sagnac scale factor by using a short length coil and restricting the maximum input rate. The operating regime is selected to be near the zero of the sine function at the fundamental phase modulation frequency. For a small modulation depth, the sine function can be approximated by a straight line. The amplitude of the second harmonic of the phase modulation frequency is then at the peak of a cosine function and consequently varies very little with rotation rate. The peak value of the second harmonic can then be used as a measure of the detected signal intensity to control and maintain the light source power over time and operating temperature. Thus, by appropriately selecting the modulation depth to minimize the sensitivity of the second harmonic to rotation, the PZT can be operated at approximately the same modulation depth over a range of temperature. With these design criteria, analog signal processing can provide a rate sensor with excellent performance. However, controlling the scale factor over an extended period of time and a wide range of operating temperatures by using the level of the second harmonic signal alone is a challenge for analog processing electronics.
It would therefore be desirable to provide an analog signal processing system and method for a fiber optic sensor that improves the environmental and temporal stability of the sensor scale factor.
The invention is directed to an analog signal processing system for fiber optic sensors. In particular, the digital signal processing system described herein can be used to stabilize the sensor scale factor. The fiber optic sensor coil can measure physical quantities, such as a rotation rate or a magnetic field. In one aspect, the present invention provides a method of controlling the scale factor of a fiber optic sensor comprising amplitude modulating the signal driving a phase modulator in a fiber optic sensor, and using the component of the detected signal at the amplitude modulation frequency to control the laser power in the sensor. As will be apparent in the following discussion, this method has the advantage of stabilizing the scale factor and minimizing the component of the detected signal at twice the amplitude modulation frequency. In another aspect, the present invention provides a method of controlling the scale factor my maximizing the component of the detected signal at the amplitude modulation frequency. In still another aspect, the present invention provides a fiber optic sensor in which the scale factor is stabilized by using the component of the detected signal at the amplitude modulation frequency to control the power level of an optical source.
Further features and advantages of the present invention will be apparent from the following description of preferred embodiments and from the claims.